Electronic packages commonly employ a plurality of surface mount electronic devices such as diodes, inductors, capacitors, resistors, varisters, etc., assembled onto a printed circuit board. The printed circuit board generally includes a dielectric substrate (e.g. organic resin reinforced by fibers) and multiple layers of electrically conductive circuit traces. Many circuit boards include perforations (holes) for matingly receiving lead lines on lead type surface mount electronic devices that form an electrical connection to the circuit traces.
Lead-less surface mount devices have also been mounted onto printed circuit boards by using surface formed solder joints. However, conventional lead-less surface mount devices have been known to suffer from thermal fatigue in the solder joint, particularly when large surface mount devices are mounted on an organic circuit board and utilized in an environment with high temperature (e.g., +100° C.) and/or wide temperature variations (e.g., −40° C. to +150° C.). The solder joint fatigue is at least partially caused by large differences in the differential coefficients of thermal expansion (CTE) that exist between the circuit board and the surface mount device materials. These differences in thermal expansion can result in catastrophic cracking of brittle components such as surface mount capacitors. Generally, larger components have higher stress and, thus, shorter component life. However, large components are generally desirable because fewer components are required.
Surface mount devices typically have much smaller coefficients of expansion as compared to organic based substrates employed in the circuit board. Temperature fluctuations of the electronic package with continuous power cycles generally produce accumulative fatigue in the solder joints. This accumulative thermal fatigue produces intergranular precipitation and alloy separation in the solder joints which accelerates component breakage. The solder joint fatigue may be accelerated by the presence of vibrations. Additionally, the surface mount devices are typically pulled down tightly to the mounting pads by the action of gravity, soldering, and capillary attraction, thereby resulting in very low collumar compliance. This may result in catastrophic electrical failure of the package due to breakage of the solder joint and/or surface mount device.
Several approaches have been proposed to elevate the surface mount device from the circuit board. According to one approach, mechanical spacers are disposed between the surface mount device and the circuit board and separate from the solder connections to elevate the surface mount device from the circuit board. According to another approach, high temperature solder stand-off members are disposed in the solder paste such that during reflow, the high temperature solder stand-off members remain solid (rigid), and thus provide a stand-off height to elevate the surface mount device from the circuit board. While some approaches elevate the surface mount device from the circuit board, many approaches typically elevate the surface mount device by more than 0.25 mm which may leave the device and solder joint susceptible to damage caused by vibration, particularly at the harmonic frequency of the resultant structure. Extensive elevation of the surface mount device from the circuit board may lead to excessive movement of the device relative to the circuit board and, hence, may weaken the solder joint and lead to breakage of the interconnecting solder joint.
Accordingly, it is therefore desirable to provide for an electronic package having a surface mount device to circuit board interconnection which is less susceptible to thermal fatigue. In particular, it is desirable to provide for such an electronic package that allows for the use of one or more large surface mount devices on a circuit board, that is less susceptible to adverse effects (e.g., breakage) caused by variations in the thermal coefficients of expansion of the materials. It is further desirable to optimize the elevation of the surface mount device from the circuit board to minimize vibration induced fatigue of the solder joint.